Information storage systems are employed to store data which are processed by data processors such as micro-computers, mini-computers and main frames. A disk drive is one type of information storage system whose use is rapidly expanding, in part because it provides high-capacity, on-line, random access storage for small to medium scale computer systems. The data storage medium of the disk drive can be a hard, i.e., inflexible, disk or a soft, i.e., flexible, disk or diskette, known as a floppy disk. The disk drive has a read/write recording head that is movable radially across concentric data tracks on the diskette while the diskette is rotated on a spindle. Disk drives are identified by their sizes and given generic names such as mini-drives and maxi-drives.
As can be appreciated, a wide variety of technical problems must be satisfactorily solved before a disk drive can be adequately utilized. One problem is to provide a disk drive of a given size with a higher, on-line, data storage capacity. One type of disk drive solves this problem by providing a disk pack which stores a plurality of diskettes. A drive mechanism linearly moves the disk pack up and down to bring any one of the diskettes into a retrievable position. The one diskette then is retrieved or picked from the disk pack and moved inwardly to an operative or rotational position on the spindle. After use, the diskette is returned to the disk pack and the procedure can then be repeated to retrieve another diskette.
While the storage of a plurality of diskettes in the disk pack increases the data storage capacity of the disk drive, the linear up and down motion of the pack adds to the space requirements of the disk drive. Furthermore, a relatively complicated drive mechanism is utilized to move the disk pack up and down. Also, this space requirement and complicated drive mechanism increase the cost of the disk drive.
Another problem relates to the design of a picker mechanism for retrieving the diskette from, and returning it to, the disk pack. A prior picker mechanism has spring-biased jaws which are driven onto an edge of the diskette to open slightly and clamp the diskette. Then, the jaws hold the diskette by friction while the diskette is moved to the operative position. Thereafter, the jaws slide off the diskette to unclamp or release the diskette in the operative position. One disadvantage is that the jaws are clamped on the jacket of the diskette, which can cause wear of the jacket and damage to the data storage medium in the jacket. Also, relatively high electrical power and a large motor are required to drive the jaws onto the diskette edge. Furthermore, the power needed to drive the jaws onto the edge must be balanced against the power used in moving the jaws towards the operative position to prevent the jaws from slipping off the edge.
Yet another problem is the expansion or contraction exhibited by the data storage medium in response to temperature and humidity conditions. This expansion or contraction increases or decreases the distance between centerlines of the concentric data tracks, causing problems in accurately positioning or servoing the recording head over a data track. One prior disk drive uses an outer and an inner servo track being outside and inside the data tracks, respectively. The expansion and contraction of the data storage medium are compensated by measuring the variable distance between the inner and outer servo tracks, and then assigning proportional locations for all the data tracks which lie between the servo tracks. As the recording head is moved to a particular data track, a scale is sensed to detect the distance moved by the head. When the distance moved by the head indicates that the assigned position has been reached, the head is assumed to be centered over the particular data track and movement of the head is stopped.
One disadvantage with the above compensation scheme is that two servo tracks must be written on the storage medium by the diskette manufacturer. The use of two servo tracks also reduces the available data storage capacity of the diskette. Another disadvantage is that the recording head must be moved across the diskette between the inner and outer servo tracks to detect their relative positions and calculate the distance between them, thereby requiring a relatively long time for collecting the data to make the compensation. Still another disadvantage is the use of a complicated algorithm for computing the data assigning positions to the data tracks.